1. Field of the Invention (Technical Field)
The invention generally relates to the composition and fabrication of thin films of tungsten-doped barium strontium titanate.
2. Background Art
Thin ferroelectric films have application as thin-film capacitor devices, as memory devices utilizing ferroelectric memories, including dynamic random access memory (xe2x80x9cDRAMxe2x80x9d) devices, and for related applications. Ferroelectric materials are also being widely used in the development of new microwave devices such as frequency agile filters, phase shifters, and tunable high-Q resonators.
The utility of thin-film ferroelectric materials is directly related to the dielectric constant of the material and the dielectric loss or rate of leakage of the material. For example, the size of the capacitor used in a DRAM integrated circuit is the predominant element that determines the size of a DRAM cell. Thus, utilizing a thin-film capacitor that has either a higher dielectric constant or a lower loss may decrease the size of a DRAM cell. All else being equal, a capacitor with a high dielectric constant and a high loss is not as useful as a capacitor with a slightly lower dielectric constant coupled with a substantially lower loss. Thus, neither the dielectric constant nor the dielectric loss is an absolute measure of the utility of the thin-film ferroelectric materials, but utility is rather determined by the ratio of the dielectric constant to the loss, and the requirements for the specific application. For example, in DRAM applications, if lower power consumption is desired, such as might be obtained by a decreased reflashing or refreshing rate, then the loss is more critical than the dielectric constant.
Thus thin-film ferroelectric materials may be improved by either increasing the dielectric constant of the material (provided that the loss does not correspondingly increase), or by decreasing the loss of the material (provided that the dielectric constant does not correspondingly decrease). Many prior methods for increasing the dielectric constant of material have also increased the leakage current of the material. Some methods have sought to increase the dielectric constant of materials, even for high dielectric constant materials, without significantly increasing the leakage current. However, substantially less attention has been paid to producing high dielectric constant thin-film ferroelectric materials with decreased losses.
Metal oxide materials, including barium strontium titanate (xe2x80x9cBSTxe2x80x9d), have been used as high dielectric constant thin-film ferroelectric materials. These materials, of the general composition BaSrTiO3, have been intensely studied for such applications due to their comparatively low loss and high dielectric constants. Methods and compositions for utilizing BST materials are taught generally in U.S. Pat. No. 5,723,361 to Azuma and others, teaching methods to increase the dielectric constant of BST materials with little or no effect on leakage current; U.S. Pat. No. 5,853,500 to Joshi and Paz de Araujo, teaching methods of fabricating high dielectric constant BST materials; U.S. Pat. No. 5,874,379 to Joo and Joo, teaching improved BST dielectric films; and U.S. Pat. No. 5,889,696 to Kawakubo and others, teaching uses for BST materials. However, there remains a need for materials that have lower losses than current BST thin-film materials while maintaining dielectric constants that are not significantly lower than the dielectric constants of current BST thin-film materials.
There has not been a thorough and systematic study of the effects of dopants on the properties of BST thin-film materials, including use of dopants to create thin-film materials with a lower loss while maintaining acceptable dielectric constants, particularly in the microwave frequencies. Development and optimization of new dielectric or ferroelectric materials for microwave applications conventionally involves complicated materials synthesis and difficult microwave characterization measurements, and thus has not been undertaken in any systematic manner.
There is provided a dielectric thin film comprising (BaxSr1xe2x88x92x)TiO3 doped with W, wherein X is between about 0.5 and 1.0. In this dielectric thin film, W may be 1 mol % or greater, based on the total molarity of Ba, Sr and Ti. W may be elemental W, or may be in the form WO3. In this dielectric thin film, the loss tangent of the film doped with W is substantially lower than the loss tangent of a film of the same composition but not doped with W.
There is also provided a dielectric thin film comprising BaTiO3 doped with W. In this dielectric thin film, W may be 1 mol % or greater, based on the total molarity of Ba and Ti. W may be elemental W, or may be in the form WO3. In this dielectric thin film, the loss tangent of the film doped with W is substantially lower than the loss tangent of a film of the same composition but not doped with W.
There is also provided a method of forming a dielectric thin film consisting of (BaxSr1xe2x88x92x)TiO3 doped with W, wherein X is between about 0.5 and 1.0. This method includes the steps of: providing a substrate; depositing TiO2, Ba, Sr and W on the substrate; and heating the substrate containing deposited TiO2, Ba, Sr and W to form the thin film. In this method, the substrate may be a single crystal, and the substrate may further be LaAlO3 or MgO. In this method, X may be 1, such that no Sr is deposited. W may be in any of a variety of forms, including elemental W and WO3. In this method, the heating step may be conducted in the presence of flowing oxygen, may be at a temperature of at least 400xc2x0 C., and may be for a period of at least 24 hours. In this method, there may also be a further step of annealing the substrate containing the thin film subsequent to the heating step. The further annealing may be at a temperature of at least 900xc2x0 C., may be in the presence of flowing oxygen, and may be for a period of at least one and one-half hours.
A primary object of the present invention is to provide a BST thin-film material which, by reason of containing a tungsten dopant, has substantially lower dielectric loss or leakage, while maintaining an acceptable dielectric constant.
A further object of the present invention is to provide a barium titanate thin-film material which, by reason of containing a tungsten dopant, has substantially low dielectric loss or leakage, while maintaining an acceptable dielectric constant.
A further object of the present invention is to provide a thin ferroelectric film with a low dielectric loss or leakage and an acceptable dielectric constant, and having enhanced application, particularly for high-frequency or microwave applications, for uses such as DRAM, frequency agile filter, phase shifter, and tunable high-Q resonator devices.
A further object of the present invention is to provide a method for fabrication of thin ferroelectric BST or barium titanate films doped with tungsten, having a characteristic of a decreased leakage current.
A primary advantage of the present invention is that the low dielectric loss or leakage of the compositions of this invention permit the manufacture of smaller devices, including memory devices such as DRAM devices, and devices such as frequency agile filters, phase shifters, and tunable high-Q resonators.
Yet another advantage of the present invention is that the low dielectric loss or leakage of the compositions of this invention permit the manufacture of devices with a lower power consumption requirement, including memory devices such as DRAM devices, and devices such as frequency agile filters, phase shifters, and tunable high-Q resonators.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.